1. Field of the Invention
The present invention relates to a photomask, more particularly to a photomask that can improve the level of process tolerance and compensate for micro-regional defocus effect on a wafer.
2. Description of the Related Art
With increase of the density of circuit components of electronic products, semiconductor lithography technique has been developed to meet the requirement of production of finer images. Increasing the depth of focus (DOF) so as to increase lithography process window facilitates reduction of reproducibility and yield loss. Improvement in resolution is an important subject in lithography techniques. There are many ways to improve the resolution, for example, employing a lens with a larger numerical aperture, or employing an exposure light with a shorter wavelength, e.g., deep ultraviolet KrF (wavelength=248 nm) and ArF (wavelength=193 nm) used to replace conventional I-line (wavelength=365 nm). In addition, the resolution may be enhanced by, for example, a phase shift masking technique, an off-axis illumination technique, and the like.
Generally, in the manufacturing process of semiconductor elements, a plurality of masking steps are required to pattern a photoresist layer on a wafer so as to form, e.g. a mask used in an etching or ion implantation step. Referring to FIG. 1, a conventional binary mask 1 with different wiring densities comprises a glass substrate 11 and a predetermined pattern 12. The predetermined pattern 12 comprises a plurality of spaced apart chromium light-blocking layers 121 having a substantially identical thickness, a plurality of light transmissive regions 122A, 122B each of which is definedby two adjacent ones of the light-blocking layers 121 and in which the light transmissive regions 122B have a larger width than that of the light transmissive regions 122A, and a plurality of anti-reflection layers 123 each of which is formed on a surface of the corresponding one of the light-blocking layers 121 opposite to the substrate 11. After the predetermined pattern 12 is formed on the glass substrate 11, the predetermined pattern 12 may be transferred to the photoresist layer on the wafer via an exposure process. The exposed photoresist layer is then subjected to post exposure baking, developing, and etching steps to form a specific wiring structure. An image of the predetermined pattern 12 on the mask 1 is formed on a focal plane on the opposite side of the lens when the exposure light radiates the predetermined pattern 12. When the focal plane overlaps with an optimum photoresist plane, a photoresist pattern with the optimum resolution can thus be obtained. However, when the exposure light passes through the light transmissive regions 122A, 122B with different widths (i.e., different wiring densities) and radiates the photoresist, the photoresist layer will produce different concentrations of acid cation on the focal plane due to the poor quality of the focal lens or the different wiring densities of the mask 1. Therefore, when the photoresist layer is subjected to a subsequent process such as the post exposure baking, due to the diffusion difference of the acid cation, the photoresist pattern formed with different wiring densities may have problems e.g., incomplete development, over development, undercut or the like, which results in a resolution problem. On the other hand, since the thickness of the photoresist layer on the wafer may not be uniform due to the uneven wafer surface, and the exposure light passing through the mask which is formed with the chromium light-blocking layers 121 having a substantially identical thickness may focus on a same horizontal plane of the photoresist layer after the exposure process, for the photoresist layer with different thicknesses, the photoresist pattern thus formed is liable to have a non-uniform resolution problem or focal depth variation or defocus effect when the mask 1 is aligned with the photoresist layer, thereby causing unqualified critical dimension, poor line edge roughness, or inferior cross sectional profile of the photoresist pattern. It is understood that exposure amount and focusing control are important parameters for the distribution quality of the photoresist pattern on the wafer, and the defocus problem may usually result in reduced process window and increased difficulty in the process. The defocus problem during the light exposure process is attributed to the following defocus factors: lens aberration, vibration during the exposure process, inclination of the wafer or mask on a platform, or the non-uniform flatness of a wiring layer on the wafer.
To solve the defocus problem, conventionally, one solution is to improve the lithography equipment, and another is to build up a system that is capable of instantly detecting and feedback correcting the defocus factors. In addition, in the present semiconductor manufacturing process, before performing the lithography exposure process, a product wafer is selected to perform the measurement of the focus-exposure matrices so as to decide the optimum process focus value. However, it is still desired to provide a better way to improve the poor development resolution due to the wiring density difference or non-uniform flatness of structure on the same chip or to compensate the micro-regional defocus effect.